The lifespan of any vessel, barge, ship, ferry, whatever that's built from steel and lives its entire life in saltwater is limited. I don't think anyone should be surprised that something built in 1962 has become uneconomical to maintain and needs to be scrapped.
In this case it's probably unique enough that someone did the math on it and determined that for however many millions of euros are being spent to rehabilitate it in a shipyard, keeping it viable for another 10-15 years, it's less expensive than building an entirely new one to a custom design.
Not true. A steel boat can last indefinitely if regularly maintained. Properly painted steel does not rust. There are many surviving examples from the 1830s, not restored but maintained continuously.
Properly painted steel wont. But steel wont stay "properly painted" during operation, leading to rust forming between maintenance. Maintenance is also typically rushed leading to painting over rust in many cases resulting in more rust sooner.
We used to joke about the local ferries being mostly rust and paint, but it wasn't much of a joke to be honest.
It's not that it's uneconomical to maintain the core ship. It's that ships periodically need to be refitted the same way houses get renovated and without a future use to justify that there's no reason to do so.
I've seen detailed photo galleries of former WA state ferries when they go for auction, after they've reached the end of their service life as judged by the state government. Usually at the 40+ year mark. Throughout their service lives they get refitted and fixed up on an almost continual basis, many millions of dollars are spent on maintaining each one, but at a certain point, it starts looking like a money pit to pour funds into continuing to fix up a 35, 40 year old vessel in salt water.
There's some ships on the great lakes which are 70, 75, 80 years old and don't have nearly the same ongoing corrosion issues as similar ones that live in salt water.